^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 1) Using the initial RAM disk (initrd)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 2) ===================================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 3)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 4) Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 5) Hans Lermen <lermen@fgan.de>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 6)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 7)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 8) initrd provides the capability to load a RAM disk by the boot loader.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 9) This RAM disk can then be mounted as the root file system and programs
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10) can be run from it. Afterwards, a new root file system can be mounted
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11) from a different device. The previous root (from initrd) is then moved
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12) to a directory and can be subsequently unmounted.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14) initrd is mainly designed to allow system startup to occur in two phases,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15) where the kernel comes up with a minimum set of compiled-in drivers, and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) where additional modules are loaded from initrd.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18) This document gives a brief overview of the use of initrd. A more detailed
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) discussion of the boot process can be found in [#f1]_.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22) Operation
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23) ---------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25) When using initrd, the system typically boots as follows:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) 1) the boot loader loads the kernel and the initial RAM disk
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28) 2) the kernel converts initrd into a "normal" RAM disk and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29) frees the memory used by initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30) 3) if the root device is not ``/dev/ram0``, the old (deprecated)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31) change_root procedure is followed. see the "Obsolete root change
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) mechanism" section below.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) 4) root device is mounted. if it is ``/dev/ram0``, the initrd image is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34) then mounted as root
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) 5) /sbin/init is executed (this can be any valid executable, including
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36) shell scripts; it is run with uid 0 and can do basically everything
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37) init can do).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) 6) init mounts the "real" root file system
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39) 7) init places the root file system at the root directory using the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40) pivot_root system call
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41) 8) init execs the ``/sbin/init`` on the new root filesystem, performing
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42) the usual boot sequence
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43) 9) the initrd file system is removed
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45) Note that changing the root directory does not involve unmounting it.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46) It is therefore possible to leave processes running on initrd during that
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47) procedure. Also note that file systems mounted under initrd continue to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 48) be accessible.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 49)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 50)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 51) Boot command-line options
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 52) -------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 53)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 54) initrd adds the following new options::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 55)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 56) initrd=<path> (e.g. LOADLIN)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 57)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 58) Loads the specified file as the initial RAM disk. When using LILO, you
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 59) have to specify the RAM disk image file in /etc/lilo.conf, using the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 60) INITRD configuration variable.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 61)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 62) noinitrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 63)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 64) initrd data is preserved but it is not converted to a RAM disk and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 65) the "normal" root file system is mounted. initrd data can be read
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 66) from /dev/initrd. Note that the data in initrd can have any structure
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 67) in this case and doesn't necessarily have to be a file system image.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 68) This option is used mainly for debugging.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 69)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 70) Note: /dev/initrd is read-only and it can only be used once. As soon
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 71) as the last process has closed it, all data is freed and /dev/initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 72) can't be opened anymore.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 73)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 74) root=/dev/ram0
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 75)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 76) initrd is mounted as root, and the normal boot procedure is followed,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 77) with the RAM disk mounted as root.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 78)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 79) Compressed cpio images
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 80) ----------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 81)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 82) Recent kernels have support for populating a ramdisk from a compressed cpio
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 83) archive. On such systems, the creation of a ramdisk image doesn't need to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 84) involve special block devices or loopbacks; you merely create a directory on
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 85) disk with the desired initrd content, cd to that directory, and run (as an
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 86) example)::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 87)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 88) find . | cpio --quiet -H newc -o | gzip -9 -n > /boot/imagefile.img
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 89)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 90) Examining the contents of an existing image file is just as simple::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 91)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 92) mkdir /tmp/imagefile
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 93) cd /tmp/imagefile
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 94) gzip -cd /boot/imagefile.img | cpio -imd --quiet
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 95)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 96) Installation
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 97) ------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 98)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 99) First, a directory for the initrd file system has to be created on the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100) "normal" root file system, e.g.::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 101)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 102) # mkdir /initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 103)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 104) The name is not relevant. More details can be found on the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 105) :manpage:`pivot_root(2)` man page.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 106)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 107) If the root file system is created during the boot procedure (i.e. if
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 108) you're building an install floppy), the root file system creation
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 109) procedure should create the ``/initrd`` directory.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 110)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 111) If initrd will not be mounted in some cases, its content is still
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 112) accessible if the following device has been created::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 113)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 114) # mknod /dev/initrd b 1 250
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 115) # chmod 400 /dev/initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 116)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 117) Second, the kernel has to be compiled with RAM disk support and with
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 118) support for the initial RAM disk enabled. Also, at least all components
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 119) needed to execute programs from initrd (e.g. executable format and file
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 120) system) must be compiled into the kernel.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 121)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 122) Third, you have to create the RAM disk image. This is done by creating a
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 123) file system on a block device, copying files to it as needed, and then
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 124) copying the content of the block device to the initrd file. With recent
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 125) kernels, at least three types of devices are suitable for that:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 126)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 127) - a floppy disk (works everywhere but it's painfully slow)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 128) - a RAM disk (fast, but allocates physical memory)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 129) - a loopback device (the most elegant solution)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 130)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 131) We'll describe the loopback device method:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 132)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 133) 1) make sure loopback block devices are configured into the kernel
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 134) 2) create an empty file system of the appropriate size, e.g.::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 135)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 136) # dd if=/dev/zero of=initrd bs=300k count=1
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 137) # mke2fs -F -m0 initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 138)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 139) (if space is critical, you may want to use the Minix FS instead of Ext2)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 140) 3) mount the file system, e.g.::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 141)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 142) # mount -t ext2 -o loop initrd /mnt
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 143)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 144) 4) create the console device::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 145)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 146) # mkdir /mnt/dev
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 147) # mknod /mnt/dev/console c 5 1
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 148)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 149) 5) copy all the files that are needed to properly use the initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 150) environment. Don't forget the most important file, ``/sbin/init``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 151)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 152) .. note:: ``/sbin/init`` permissions must include "x" (execute).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 153)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 154) 6) correct operation the initrd environment can frequently be tested
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 155) even without rebooting with the command::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 156)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 157) # chroot /mnt /sbin/init
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 158)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 159) This is of course limited to initrds that do not interfere with the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 160) general system state (e.g. by reconfiguring network interfaces,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 161) overwriting mounted devices, trying to start already running demons,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 162) etc. Note however that it is usually possible to use pivot_root in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 163) such a chroot'ed initrd environment.)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 164) 7) unmount the file system::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 165)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 166) # umount /mnt
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 167)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 168) 8) the initrd is now in the file "initrd". Optionally, it can now be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 169) compressed::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 170)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 171) # gzip -9 initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 172)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 173) For experimenting with initrd, you may want to take a rescue floppy and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 174) only add a symbolic link from ``/sbin/init`` to ``/bin/sh``. Alternatively, you
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 175) can try the experimental newlib environment [#f2]_ to create a small
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 176) initrd.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 177)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 178) Finally, you have to boot the kernel and load initrd. Almost all Linux
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 179) boot loaders support initrd. Since the boot process is still compatible
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 180) with an older mechanism, the following boot command line parameters
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 181) have to be given::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 182)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 183) root=/dev/ram0 rw
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 184)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 185) (rw is only necessary if writing to the initrd file system.)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 186)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 187) With LOADLIN, you simply execute::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 188)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 189) LOADLIN <kernel> initrd=<disk_image>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 190)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 191) e.g.::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 192)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 193) LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0 rw
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 194)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 195) With LILO, you add the option ``INITRD=<path>`` to either the global section
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 196) or to the section of the respective kernel in ``/etc/lilo.conf``, and pass
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 197) the options using APPEND, e.g.::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 198)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 199) image = /bzImage
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 200) initrd = /boot/initrd.gz
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 201) append = "root=/dev/ram0 rw"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 202)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 203) and run ``/sbin/lilo``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 204)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 205) For other boot loaders, please refer to the respective documentation.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 206)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 207) Now you can boot and enjoy using initrd.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 208)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 209)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 210) Changing the root device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 211) ------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 212)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 213) When finished with its duties, init typically changes the root device
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 214) and proceeds with starting the Linux system on the "real" root device.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 215)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 216) The procedure involves the following steps:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 217) - mounting the new root file system
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 218) - turning it into the root file system
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 219) - removing all accesses to the old (initrd) root file system
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 220) - unmounting the initrd file system and de-allocating the RAM disk
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 221)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 222) Mounting the new root file system is easy: it just needs to be mounted on
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 223) a directory under the current root. Example::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 224)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 225) # mkdir /new-root
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 226) # mount -o ro /dev/hda1 /new-root
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 227)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 228) The root change is accomplished with the pivot_root system call, which
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 229) is also available via the ``pivot_root`` utility (see :manpage:`pivot_root(8)`
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 230) man page; ``pivot_root`` is distributed with util-linux version 2.10h or higher
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 231) [#f3]_). ``pivot_root`` moves the current root to a directory under the new
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 232) root, and puts the new root at its place. The directory for the old root
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 233) must exist before calling ``pivot_root``. Example::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 234)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 235) # cd /new-root
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 236) # mkdir initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 237) # pivot_root . initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 238)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 239) Now, the init process may still access the old root via its
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 240) executable, shared libraries, standard input/output/error, and its
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 241) current root directory. All these references are dropped by the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 242) following command::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 243)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 244) # exec chroot . what-follows <dev/console >dev/console 2>&1
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 245)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 246) Where what-follows is a program under the new root, e.g. ``/sbin/init``
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 247) If the new root file system will be used with udev and has no valid
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 248) ``/dev`` directory, udev must be initialized before invoking chroot in order
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 249) to provide ``/dev/console``.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 250)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 251) Note: implementation details of pivot_root may change with time. In order
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 252) to ensure compatibility, the following points should be observed:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 253)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 254) - before calling pivot_root, the current directory of the invoking
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 255) process should point to the new root directory
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 256) - use . as the first argument, and the _relative_ path of the directory
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 257) for the old root as the second argument
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 258) - a chroot program must be available under the old and the new root
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 259) - chroot to the new root afterwards
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 260) - use relative paths for dev/console in the exec command
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 261)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 262) Now, the initrd can be unmounted and the memory allocated by the RAM
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 263) disk can be freed::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 264)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 265) # umount /initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 266) # blockdev --flushbufs /dev/ram0
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 267)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 268) It is also possible to use initrd with an NFS-mounted root, see the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 269) :manpage:`pivot_root(8)` man page for details.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 270)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 271)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 272) Usage scenarios
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 273) ---------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 274)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 275) The main motivation for implementing initrd was to allow for modular
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 276) kernel configuration at system installation. The procedure would work
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 277) as follows:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 278)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 279) 1) system boots from floppy or other media with a minimal kernel
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 280) (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 281) loads initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 282) 2) ``/sbin/init`` determines what is needed to (1) mount the "real" root FS
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 283) (i.e. device type, device drivers, file system) and (2) the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 284) distribution media (e.g. CD-ROM, network, tape, ...). This can be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 285) done by asking the user, by auto-probing, or by using a hybrid
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 286) approach.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 287) 3) ``/sbin/init`` loads the necessary kernel modules
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 288) 4) ``/sbin/init`` creates and populates the root file system (this doesn't
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 289) have to be a very usable system yet)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 290) 5) ``/sbin/init`` invokes ``pivot_root`` to change the root file system and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 291) execs - via chroot - a program that continues the installation
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 292) 6) the boot loader is installed
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 293) 7) the boot loader is configured to load an initrd with the set of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 294) modules that was used to bring up the system (e.g. ``/initrd`` can be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 295) modified, then unmounted, and finally, the image is written from
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 296) ``/dev/ram0`` or ``/dev/rd/0`` to a file)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 297) 8) now the system is bootable and additional installation tasks can be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 298) performed
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 299)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 300) The key role of initrd here is to re-use the configuration data during
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 301) normal system operation without requiring the use of a bloated "generic"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 302) kernel or re-compiling or re-linking the kernel.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 303)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 304) A second scenario is for installations where Linux runs on systems with
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 305) different hardware configurations in a single administrative domain. In
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 306) such cases, it is desirable to generate only a small set of kernels
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 307) (ideally only one) and to keep the system-specific part of configuration
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 308) information as small as possible. In this case, a common initrd could be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 309) generated with all the necessary modules. Then, only ``/sbin/init`` or a file
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 310) read by it would have to be different.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 311)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 312) A third scenario is more convenient recovery disks, because information
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 313) like the location of the root FS partition doesn't have to be provided at
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 314) boot time, but the system loaded from initrd can invoke a user-friendly
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 315) dialog and it can also perform some sanity checks (or even some form of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 316) auto-detection).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 317)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 318) Last not least, CD-ROM distributors may use it for better installation
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 319) from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 320) via initrd from CD; or by booting via a loader like ``LOADLIN`` or directly
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 321) from the CD-ROM, and loading the RAM disk from CD without need of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 322) floppies.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 323)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 324)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 325) Obsolete root change mechanism
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 326) ------------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 327)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 328) The following mechanism was used before the introduction of pivot_root.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 329) Current kernels still support it, but you should _not_ rely on its
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 330) continued availability.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 331)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 332) It works by mounting the "real" root device (i.e. the one set with rdev
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 333) in the kernel image or with root=... at the boot command line) as the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 334) root file system when linuxrc exits. The initrd file system is then
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 335) unmounted, or, if it is still busy, moved to a directory ``/initrd``, if
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 336) such a directory exists on the new root file system.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 337)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 338) In order to use this mechanism, you do not have to specify the boot
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 339) command options root, init, or rw. (If specified, they will affect
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 340) the real root file system, not the initrd environment.)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 341)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 342) If /proc is mounted, the "real" root device can be changed from within
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 343) linuxrc by writing the number of the new root FS device to the special
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 344) file /proc/sys/kernel/real-root-dev, e.g.::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 345)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 346) # echo 0x301 >/proc/sys/kernel/real-root-dev
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 347)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 348) Note that the mechanism is incompatible with NFS and similar file
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 349) systems.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 350)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 351) This old, deprecated mechanism is commonly called ``change_root``, while
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 352) the new, supported mechanism is called ``pivot_root``.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 353)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 354)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 355) Mixed change_root and pivot_root mechanism
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 356) ------------------------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 357)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 358) In case you did not want to use ``root=/dev/ram0`` to trigger the pivot_root
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 359) mechanism, you may create both ``/linuxrc`` and ``/sbin/init`` in your initrd
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 360) image.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 361)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 362) ``/linuxrc`` would contain only the following::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 363)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 364) #! /bin/sh
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 365) mount -n -t proc proc /proc
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 366) echo 0x0100 >/proc/sys/kernel/real-root-dev
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 367) umount -n /proc
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 368)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 369) Once linuxrc exited, the kernel would mount again your initrd as root,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 370) this time executing ``/sbin/init``. Again, it would be the duty of this init
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 371) to build the right environment (maybe using the ``root= device`` passed on
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 372) the cmdline) before the final execution of the real ``/sbin/init``.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 373)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 374)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 375) Resources
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 376) ---------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 377)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 378) .. [#f1] Almesberger, Werner; "Booting Linux: The History and the Future"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 379) https://www.almesberger.net/cv/papers/ols2k-9.ps.gz
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 380) .. [#f2] newlib package (experimental), with initrd example
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 381) https://www.sourceware.org/newlib/
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 382) .. [#f3] util-linux: Miscellaneous utilities for Linux
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 383) https://www.kernel.org/pub/linux/utils/util-linux/
Orange Pi5 kernel
Deprecated Linux kernel 5.10.110 for OrangePi 5/5B/5+ boards
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